Therapeutic drug monitoring (TDM), or simply drug monitoring, is the measurement of drug levels in the blood.
TDM is employed to measure blood drug levels so that the most effective dosage can be determined and toxicity can be prevented. Drug monitoring is not needed for most drugs. Many drugs have a wide therapeutic window, meaning that the difference between the therapeutic and toxic level is large. Often, the physician can measure an expected outcome to see if a drug is working. For example, body temperature can be measured to evaluate an antipyretic drug. Monitoring is mainly used for drugs that can be toxic or cause severe side effects. Examples are antiepileptic drugs, antiarrhythmic agents, oral anticoagulants, theophylline, tricyclic antidepressants, lithium, antineoplastics, aminoglycosideantibiotics, cardiac glycosides, and drugs to prevent transplant rejection. TDM is also utilized to identify noncompliant patients (i.e., those patients who, for whatever reason, either cannot or will not comply with drug dosages as prescribed by the physician).
Many different factors influence blood drug levels, and the following points should be taken into consideration during TDM: the age, sex, and weight of the patient; the route of administration of the drug; the drug's absorption rate, excretion rate, delivery rate, and dosage; other medications the patient is taking; other diseases the patient has; the patient's compliance regarding the drug treatment regimen; and the laboratory methods used to measure the drug.
Drugs taken orally should not be measured until the processes of absorption and elimination have nearly reached a steady state. The steady state is reached when the drug in the next dose is sufficient to replace the drug that is eliminated. This requires approximately five drug elimination half-lives. Some drugs such as tricyclic anti-depressants may be decreased by the gel in serum separator tubes. Since drug levels rise and then fall in between oral, bolus intravenous, and intramuscular doses, the interpretation of blood drug levels requires strict adherence to the appropriate time of collection. Blood collected at an improper time will provide misleading information. Blood should not be taken from an intravenous line immediately following infusion of medication. Before collecting a sample from an intravenous line, at least 3 mL of blood should be collected from the line and discarded to clear the line of heparin, IV contents, and medication.
Jane E. Phillips, The Gale Group Inc., Gale, Detroit,